Class: GE, II Sem Paper: Human Physiology (Theory Class) Teacher’s name: Meenakshi Rana Date: 5th May, 2020 Time: 8:30 – 10:30 AM

Structure of

Nervous tissue consists of two types of cells: and neuroglia. Neurons provide most of the unique functions of the , such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. Neuroglia support, nourish, and protect the neurons and maintain homeostasis in the interstitial fluid that bathes them.

Neurons

Neurons ( cells) possess electrical excitability, the ability to respond to a and convert it into an .

A stimulus is any change in the environment that is strong enough to initiate an action potential.

An action potential (nerve impulse) is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.

Parts of a Neuron

Most neurons have three parts:

(1) The cell body: also known as the perikaryon or , contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a

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Golgi complex. Neuronal cell bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed Nissl bodies.

A nerve fiber is a general term for any neuronal process (extension) that emerges from the cell body of a neuron. Most neurons have two kinds of processes: multiple and a single .

(2) Dendrites: They are the receiving or input portions of a neuron. They usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body. Their cytoplasm contains Nissl bodies, mitochondria, and other organelles.

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(3) An axon: An axon of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. It is a long, thin, cylindrical projection that often joins the cell body at a cone-shaped elevation called the axon hillock. The part of the axon closest to the axon hillock is the initial segment.

In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment, an area called the trigger zone, from which they travel along the axon to their destination. An axon contains mitochondria, microtubules, and neurofibrils. Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon. The cytoplasm of an axon, called , is surrounded by a plasma membrane known as the . Along the length of an axon, side branches called axon collaterals may branch off, typically at a right angle to the axon. The axon and its collaterals end by dividing into many fine processes called axon terminals (telodendria).

Classification of Neurons

Both structural and functional features are used to classify the various neurons in the body.

STRUCTURAL CLASSIFICATION

Structurally, neurons are classified according to the number of processes extending from the cell body (Figure 12.3).

1. Multipolar neurons usually have several dendrites and one axon. Most neurons in the brain and are of this type.

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2. Bipolar neurons have one main and one axon. They are found in the retina of the eye, in the inner ear, and in the olfactory area of the brain.

3. Unipolar neurons have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. These neurons are more appropriately called pseudounipolar neurons because they begin in the embryo as bipolar neurons. During development, the dendrites and axon fuse together and become a single process.

The dendrites of most unipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli. The trigger zone for nerve impulses in a is at the junction of the dendrites and axon. The impulses then propagate toward the synaptic end bulbs. The cell bodies of most unipolar neurons are located in the ganglia of spinal and cranial .

FUNCTIONAL CLASSIFICATION

Functionally, neurons are classified according to the direction in which the nerve impulse (action potential) is conveyed with respect to the CNS.

1. Sensory or afferent neurons: They either contain sensory receptors at their distal ends (dendrites) or are located just after sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure.

2. Motor or efferent neurons: They convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves. Most motor neurons are multipolar in structure.

3. or association neurons: They are mainly located within the CNS between sensory and motor neurons. Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure.

Reference: PRINCIPLES OF ANATOMY AND PHYSIOLOGY (Tortora and Derrickson)

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